The present invention relates to a method for diamond deposition directly from a gas mixture excited by a microwave plasma, said method being characterized in that a larger number of individual components can be coated under identical conditions in one single reactor run as compared to what is possible using prior art technology.
Production of diamond directly from the gas phase by CVD or PVD technique is of great interest for coating of cutting tools, drill bits, knives, etc. Gas phase deposition of diamond without formation of graphite was reported in 1977 by Derjaguin and colleagues in Moscow. When the results were reproduced by a group of Japanese scientists in the early eighties, intensive research in CVD diamond started all over the world. A series of new methods for diamond coating from gas phase has been described, including hot filament, microwave plasma, arc discharge plasma, gas flame, hollow cathode, and different forms of plasma jets. Usually pure hydrogen with the addition of 0.1 to 5.0% CH.sub.4 is used as gas mixture, but also other hydrocarbon gases can be used as carbon source. In addition, the purity and quality of the diamond film can be controlled by adding other gases, especially gases containing oxygen and/or nitrogen. Different noble gases can also be added to control the process, especially in plasma-based techniques. A relatively early patent is U.S. Pat. No. 4,707,384 that contains references to the earliest work in this area.
Low pressure deposition of diamond coatings has a large potential application in the coating of metal cutting tools like inserts, drills, end-mills, etc. However, this method has not yet been applied on a large industrial scale. One important reason for this lack of application is that the total area that can be coated under identical conditions is relatively small, frequently only a few cm in diameter using prior art technology. This is due to the fact that coating with diamond film needs excitation of the gas phase to temperatures around 2000.degree. C. At the same time, the optimum substrate temperature for diamond deposition is 1000.degree. C. or less. This leads to a strong gradient in temperature and chemical composition in the gas.
In the case of diamond deposition from a microwave plasma, the excited gas volume has a spherical or near-spherical shape. Toroid-shaped plasmas and other more complicated geometries can also be generated but they have not found any practical application since these types of plasmas tend to be unstable and therefore difficult to control during longer deposition times. Standard practice for diamond deposition is to place the components to be coated on a flat substrate table which may be equipped with auxiliary facilities for heating and/or cooling of the table. Since a microwave reactor can be considered as a resonance cavity for microwaves, it is customary to use a flat substrate table in diamond coating according to prior art to obtain a standard resonator geometry. However, the combination of a spherical plasma and a flat substrate table results in a radial variation in deposition conditions due to the variation in distance between the plasma and the table, see FIG. 1. This is valid also when the table is equipped with extra facilities for heating although in this case it may be possible to obtain a uniform temperature over a 100 mm diameter table. An additional problem with a flat table reactor design is that there is difficulty in maintaining a symmetric and stable plasma at high microwave powers.